This invention relates to my copending application entitled "Directional Power Distance Relay", Ser. No. 169,434, filed July 16, 1980 and assigned to the same assignee as the present invention, now U.S. Pat. No. 4,329,727.
This invention relates to electric power system relaying, and, more particularly, to high-speed relays for rapidly detecting multiphase faults which occur on power system transmission lines.
As power systems have grown in the power transmission industry, the time available for protective equipment to remove faults and still maintain power system stability has been significantly reduced. There are two ways of keeping pace with system stability requirements; either faster relays or faster circuit breakers are required. The instant invention is directed to reducing relay response time.
Conventional transmission line relays have reached an apparent response time limit of about 4 milliseconds, with a state-of-the-art limit of about 2.5 msec. To achieve faster response time in these conventional systems, system security would have to be compromised. System security requires that spurious operation inputs be ignored, so that system circuit breakers are not opened incorrectly during system disturbances that normally occur, but which do not interfere with system performance.
One approach to reducing relay time employs traveling wave concepts. These methods have been described by Dommel et al., "High Speed Relaying Using Traveling Wave Transient Analysis", Paper A78-214-9, IEEE PES 1978 Winter Meeting, New York, N.Y. and by Esztergalyos et al., "Development, Design, Application and Field Experience of an Ultra High Speed Relaying System for EHV/UHV Transmission Lines", Paper presented at the Pennsylvania Electric Association Relay Committee Meeting, October, 1978, Harrisburg, Pennsylvania. Because of the possibility of spurious input signals, the first wavefront to be received at the relay employing traveling wave concepts cannot be used by itself to make an operating decision. Some form of filtering, integrating or averaging of the signal must be done. The result is that the schemes respond not only to the first wavefront, but also to subsequent reflections of the first wavefront. Furthermore, since traveling waves can propagate through an entire power system, traveling wave schemes per se do not determine fault location. Although traveling wave schemes can be made to have distance properties by introducing a threshold, determining the faults magnitude precisely can be done only by extensive computer simulations. For these and other reasons including mutual coupling between adjacent lines on the same right-of-way, traveling wave schemes are not secure and are subject to generating incorrect trip signals during normal power system disturbances, such as switching surges and loss of communications channels, and can also trip the wrong circuit breakers during genuine short circuits.
Other techniques have been employed to achieve high speed relaying on transmission lines. For example, a fault simulation technique described in British Pat. No. 1,517,551, issued to Hughes Aircraft Co. on July 12, 1978, can rapidly detect single line to ground faults by comparing measured waveforms with simulated waveforms. However, the technique does not work well for multi-phase faults. As an example of a digital scheme, Takagi et al., "Digital Differential Relaying System for Transmission Line Primary Protection Using Traveling Wave Theory-Its Theory and Field Experience", Paper A-79-096-9, IEEE PES 1979 Winter Meeting, New York, N.Y., describe a method which compares the currents at opposite ends of the transmission line. However, the method cannot provide first zone distance protection without a reliable communications channel to link the line terminals.
Many conventional electronic power system relays utilize relative phase angles between signals derived from power system voltages and currents to discriminate between normal and abnormal conditions. Signals derived from voltage transformers and current transformers are combined in various ways using operations such as electronic summation, filtering, amplification, delay, squaring and thresholding. Two typical signals produced in this fashion are called polarizing voltage and operating voltage signals. A preselected angle represents the boundary between normal and abnormal power system conditions. A coincidence detector determines if the angle between the derived signals is greater or less than a critical value. This is done by measuring the time between zero crossings of the two signals. When both signals have the same sign, a timer is activated. If both signals have the same sign for a time interval greater than the timer setting, the coincidence detector generates an output. If one of the signals changes sign before the end of the preset interval, the timer is reset and no output is generated.
In a three phase power system, three or more relay circuits are needed. Although each circuit may accept inputs from all three phases, each circuit generally responds only to faults associated with the phase of pair of phases that it is protecting. Electronic logic accepts inputs from all circuits and selects the breakers that should be tripped. In some arrangements, all three phases are tripped for any type of fault. In others, one phase is tripped for a single line to ground fault and all three phases are tripped for all other types of faults.
Because the coincidence detector of conventional relays cannot produce an output until its timer times out, conventional relays have an inherent speed limitation. To some extent, conventional schemes can be made to respond more quickly by reducing the setting of the coincidence detector's timer. However, doing so moves the boundary between normal and abnormal conditions, making the relay more sensitive and less secure.
It is an object of the present invention to provide ultra high speed multiphase fault detection without any loss of relay security.
It is another object of the present invention to provide an instantaneous estimate of the relative phase angle between relay signals for multiphase fault detection.